1. Field of the Invention
The present invention generally relates to radio communication systems, and more particularly relates to a code division multiple access (CDMA) radio communication system which performs asynchronous transfer mode (ATM) communications between a base station controller and each base station.
2. Description of the Related Art
In radio communication systems, a hand-off procedure has been performed when a mobile station transfers from a service area of the first base station to an adjacent service area of the second base station. The hand-off procedure is roughly classified into a hard hand-off procedure and a soft hand-off procedure. The hard hand-off procedure is performed such that the mobile station is connected with the second base station as soon as being disconnected with the first base station. Alternatively, the soft hand-off procedure is performed such that the mobile station is always connected with both of or at least one of the first and second base stations during the hand-off transition. Accordingly, the soft hand-off procedure can be performed accurately and smoothly without dropping a call due to discontinuity of radio channels. When performing the soft hand-off procedure, the mobile station is required to comprise a rake receiver to achieve site diversity reception that assigns each radio channel between a mobile station and a plurality of base stations. In addition, in order to perform the site diversity reception, received signals transmitted from base stations via respective radio channels must be received at the same phase and include the same information. The communication quality, i.e. bit-error rate, may be improved by selecting the communication paths with low error-rates or by combining the weighted reliable communication paths.
An asynchronous transmission mode (ATM) communication can be adapted for communication between a base station controller and a base station. It is because that the ATM communication accommodates various communications transmitted/received at different transmission speeds and also allows the change in transmission speed in the middle of the call.
However, if the ATM communication is adapted between the base station controller and the base stations, from the point of view of the efficiency and the cost, it is unrealistic that the channel capacity of communication paths are provided based on the maximum traffic. In general, the channel capacity is determined in accordance with an average traffic and the maximum number of traffic channels. The channel capacity is then efficiently utilized by controlling a bandwidth of the communication paths such that signals to be transmitted are classified into two service categories, e.g.; priority traffic and non-priority traffic. Since packets on the application layer are reassembled from ATM cells on the ATM layer, the delay of packet communications tends to occur in the non-priority traffic in comparison with the priority traffic. Consequently, for example, transmission signals such as transferring data that requires no real-time transmission can be classified into non-priority traffic and transmission signals such as voice communication that requires real-time transmission can be classified into priority traffic.
The base station controller 1 performs a bandwidth-securing manner (PP1) comprised of steps RR2, RR3 and RE2 and RE3 to secure a hand-off exclusive bandwidth for packet communications. In the bandwidth-securing manner (PP1), the base station controller 1 first sends bandwidth-securing requests (RR2 and RR3) to the base stations 3 and 4 respectively. In response to the reception of the bandwidth-securing request (RR2), the bandwidth controller 22 of the base station 3 makes the soft hand-off bandwidth assign equipment 21 secure the hand-off exclusive bandwidth for the mobile station 4. Similarly, in response to the reception of the bandwidth-securing request (RR3), the bandwidth controller 22 of the base station 2 makes the soft hand-off bandwidth assign equipment 21 secure the hand-off exclusive bandwidth for the mobile station 4. The base stations 2 and 3 report a termination of securing the hand-off exclusive bandwidth to the base station controller 1 respectively (RE2 and RE3). Simultaneously, the normal communication bandwidth, which the mobile station 4 has been used to communicate with the base station 2 before the soft hand-off procedure, is canceled and released as a vacant bandwidth for other communication.
When a priority ATM cell corresponding to the priority traffic and a non-priority ATM cell corresponding to the non-priority traffic are requested to transmit at the same time, the priority ATM cell is given priority over the non-priority ATM cell. As a result, if the priority traffic is increasing on the communication path, almost all bandwidths of the communication path are assigned to the priority ATM cells, and the non-priority ATM cells are limited to transmit as to transmission speed and transmission bandwidth.
As mentioned above, to obtain the site diversity effect in the soft hand-off procedure, a plurality of transmission signals transmitted via different communication paths must be received at the same phase and include the same information. For this reason, the two base stations corresponding to the soft hand-off procedure must transmit the transmission signals at the synchronized transmission timing of transmission signals. However, it is difficult to synchronize the transmission timings of the transmission signals when the ATM communication is adapted between the base station controller and each base station. For example, when signals to be provided to two base stations for the hand-off procedure are non-priority traffic, the transmission time between the base station controller and each base station is unfixed since non-priority traffic is limited to transmit according to the amount of priority traffic. Further, since priority of service categories depends upon each base station the ATM cells that are provided to the base stations corresponding to the hand-off procedure are normally received at different timings.
When base stations receives the ATM cells at different timing and transmit radio signals corresponding to the ATM cells to the mobile station without adjusting the reception time difference, the mobile station can not perform the site diversity reception. Accordingly, the communication quality deteriorates during the hand-off procedure, or the hand-off procedure is not performed accurately and smoothly.